I know this is a truck forum, but I've found helpful info about the 400 engine / 2150 carb combo here and think you guys may be able to help. I've been working on a 1976 Mercury Marquis with a 400 and 2150 carb that has a new long block, resealed intake manifold, and rebuild carburetor. Base timing is set to 12*. Idles rough, low manifold vacuum and bouncy like a misfire. The idle screws are at 4 turns out and going further makes no improvement in the idle. Spraying the manifold and carb base with brake clean has no effect. Seems to have a small vacuum leak at throttle shaft, however. Where should I be looking now?
I'm attaching two YouTube videos of what's going on. Thanks again
-res

 

Does the idle smooth out when you change the initial timing? Are you running ported or manifold vacuum to the vacuum advance?

sure seems line you have a vacuum leak as the idle vacuum is much lower than it should be. Are you running intake gaskets or the full sheet metal pan?

is the carb new? 4 turns out is a lot but if the intake leak was really that bad and it needed 4 turns of fuel it would idle at a way higher rpm.

Have you checked which cylinders are missfireing? Maybe you have a distributor or Ignition module issue or spark plug/wire issue.

 

Do you have the EGR plate under the carb? Khown to cause problems with burned out passages.

 

Good point, It definitely came with one and it was the worst of them because it was made out of aluminum which accounts for the greatest number of passage burn out and heat deformity.

 

I had the same problem with the 2150 on my 351M. Idle screws about 4 turns out, and wouldn't change a thing adjusting. The steel throttle shaft on them is notorious for egging out the aluminum housing and causing a leak of unmetered air, that the idle adjustment screws cannot compensate for. I fixed mine by bushing the shaft with brass bushings. I can give you more info on how to do it if needed. Can even send you the reamer and some bushings.

 

Hey Buckin, thanks for the reply. The vac advance is getting ported vacuum. I just installed a new full valley pan intake gasket. The car was running worse before the new gasket (vac leak near cyl 8 head port). The carb is the original and I just rebuilt it. I disassembled it, soaked in a bucket of carb cleaner for 3 days and all ports and orifices blown out with compressed air. All vacuum ports are plugged, other than vac advance for the distributor.

New parts installed prior to me working on it: long block, distributor, plugs and wires

The ignition module box is older and leaking the black goo. Plugs look fine. When I pull one plug wire at a time while running to try to find a misfiring cylinder, I do not get a hard miss, but the rpm will drop about 200rpm. I thought maybe it was a weak spark issue, since checking spark while cranking I'm just seeing a small, seemingly weak spark and not the typical hot bright arc.

I didn't notice any bent pushrods when I had the manifold off. Maybe I can pull the valve covers and check the valve train for issues. Hydraulic lifters, so no adjustments required there.

SO, The vacuum needle is bouncing like there's a misfiring cylinder, manifold vacuum is low (10inHg), and I'm having to adjust the idle mixture way out (closing to less than 2 turns makes it run badly). That all points to a bad vacuum leak. But I don't see one, other than a small leak at the throttle shaft. Could there be a leak between an intake runner and the valley? I'm about to throw an aluminum intake and Edelbrock 650 on it.

 

Yes, the plate is still installed with the EGR valve removed and a block-off plate in its place. I'm not finding any leaks when I spray it while running. Could it be a leak between the EGR passage and throttle bore?
Can I remove the plate and bolt the carb directly to the manifold?

 

Yes you can eliminate the egr but you'll need to plug the intake and I would add a phenolic spacer equal to the thickness of the EGR plate. The only caveat is the distributor will need to be recurved in order to compensate for eliminating the egr.